Arctic acoustic tomography : MIZEX 84

Abstract

This is the final report of Contract N00014-84-C-0185 between the Woods Hole Oceanographic Institution and the Office of Naval Research for the contract period 1 January, 1984, to 28 February, 1985. This contract supported an experiment that was conducted in the Norwegian Sea during May and June of 1984 to assess the possibilities of using ocean acoustic tomography as a measuring tool in the Arctic. The object of the experiment was to identify and determine the temporal stability (coherence), and resolvability, of Arctic acoustic paths. Identification refers to the ability to match a pulse arrival with a particular ray path, primarily through computer modelling. Resolvable rays are those that arrive sufficiently far apart in time so as to be distinct and separable. In order to use tomography, rays must be identified, resolved and stable. Unlike the deep temperate ocean, where there are many wholly refracted paths, the upward refracting Arctic sound speed profile causes ray paths to reflect off the ice-covered surface of the permanent pack and the mixed ice-covered and ice-free surface of the marginal ice zones. The reflection process is time-varying and hence leads to resolvability, identification and stability questions that do not arise in the case of entirely refracted paths. A 224 Hz acoustic source was moored in an ice-free region. It transmitted phase coded, frequency stable signals to receivers fixed on the bottom and receivers drifting with the icepack at ranges of approximately 150 km. The received signals are to be analyzed with respect to identification, resolvability and stability issues. This contract covered the costs associated with installation and retrieval of the source and preliminary data reduction from the drifting and fixed hydrophones. Detailed data analysis costs are to be covered elsewhere. Nevertheless, preliminary analysis indicates that the received signals, particularly those from paths that interact with the ice-free surface, appear to have sufficient stability for tomographic purposes.